1. Field of the Invention
This invention relates to a class of aliphatic/aromatic mixed polycarbodiimides having good storage stability and particularly relates to a method for their preparation. This invention also pertains to the use of such mixed polycarbodiimides as low temperature active, e.g., "low bake", crosslinkers in aqueous, carboxyl-containing resins.
2. Description of Related Art
Carbodiimides are a well-known class of organic compounds. Dicyclohexylcarbodiimide has been used for many years as a condensation agent in the preparation of peptides, as described by Sheelan and Hess (J. Chem. Soc., 77, 1067 (1955)). Multifunctional, linear, polydisperse polycarbodiimides have been prepared by Campbell from diisocyanatoalkanes or diisocyanatoarenes using a phospholene oxide catalyst (U.S. Pat. No. 2,941,966 (1960)). The art is summarized in Chem. Rev., 81,589 (1981).
The use of polydisperse polycarbodiimides as cross-linkers for carboxylated latex resins and neutralized carboxylated water-soluble polymers is known in the art. Specifically, co-assigned U.S. application Ser. No. 691,378, filed Jan. 15, 1985 teaches the preparation of useful polycarbodiimide cross-linkers from certain mono-, di-, and tri-functional cycloaliphatic or saturated aliphatic isocyanates. Unfortunately, the raw materials used to prepare the aliphatic materials are quite expensive.
Urethane-terminated polycarbodiimides, obtained by polymerizing a diisocyanate in the presence of a carbodiimide-forming catalyst and reacting the isocyanato-terminated polymer with an alcohol, are described in U.S. Pat. No. 2,941,983 and in J. Organic Chemistry 28, 2069, (1963). All but Example 6 of U.S. Pat. No. 2,941,983 employs an equal molar amount of the diisocyanate and alcohol. In Example 6, 0.2 mol of toluene-2,4-diisocyanate is polymerized and then chain terminated using 0.14 mol of benzyl alcohol. The former reference also suggests using amines for chain growth termination; while the latter reference also discusses the production of corresponding polycarbodiimides which are terminated by reacting the isocyanato-terminated polycarbodiimide with a monoisocyanate so that the polymer contains only carbodiimide linkages. A polycarbodiimide of this type, derived from toluene diisocyanate and terminated by reaction with p-chlorophenyl isocyanate (molar ratio of diisocyanate to monoisocyanate=58), is specifically exemplified. In addition, the formation of a polycarbodiimide of unspecified molecular weight from methylenebis (phenyl isocyanate) and p-tolylisocyanate (molar ratio of diisocyanate in monoisocyanate not specified) is postulated but no preparative details or properties are given.
U.S. Pat. No. 3,450,562 teaches, in broad terms, the preparation of chain terminated polycarbodiimides by reacting, either sequentially or simultaneously, a diisocyanate and a monoisocyanate, monoalcohol or primary amine in the presence of a carbodiimide-forming catalyst. In Example I, a polymerized product of a mixture of toluene-2,4-diisocyanate and toluene-2,6-diisocyanate is chain terminated with isopropyl alcohol. In Example III, aniline is used to terminate chain growth. Example VIII of this patent shows the preparation of two mono-isocyanated terminated polycarbodiimides. The first one, polycarbodiimide 1, is obtained by heating a mixture of toluene diisocyanate and o-tolylisocyanate (in a molar ratio of 1:1) with a carbodiimide-forming catalyst. The second one, polycarbodiimide 2, is obtained by heating methylenebis (cyclohexyl isocyanate) in the presence of a carbodiimide-forming catalyst and then reacting the isocyanate-terminated polymer with cyclohexyl isocyanate. The molar ratio of diisocyanate to monoisocyanate is 1.5:1.
Co-assigned U.S. Pat. Nos. 4,487,964 and 4,587,301 disclose the preparation of useful polycabodiimide cross-linkers from mixed aromatic/aliphatic isocyanates. According to these patents, a mixed aliphatic/aromatic polycarbodiimide can be prepared by simply reacting a mixture of aliphatic mono- and diisocynates in the presence of a phospholene oxide catalyst followed, in sequence, by the addition of and further reaction with aromatic mono- or diisocyanates under similar reaction conditions. Both reactions are conducted at a temperature of about 120.degree. to 180.degree. C. and the various reactants are used in amounts to provide a molar ratio of mono- and diisocyanate reactants between about 2:1 to 2:10 and a molar ratio of aliphatic to aromatic isocyanate groups in the reactants between about 0.5:1 to 2:1.
Although this prior invention provided some improvement in polycarbodiimide resin storage stability relative to the then-existing art, excessive viscosity development and gellation over prolong storage, e.g., 6 months or longer, remains a problem in mixed polycarbodiimide resins. Due to the low level that these crosslinking materials generally are used in coating formulations, it is not uncommon for formulators to maintain these materials in inventory for that period of time. A more glaring disadvantage of this prior art invention concerns its use of monoisocyanates. The high toxicity and volatility of these materials makes them very hazardous and creates significant problems to ensure their safe handling and use.
It now has been discovered that by constructing a certain polymer structure, for example, by using a particular reaction sequence and a particular molar ratio of reactants, a mixed aliphatic/aromatic polycarbodiimide of improved storage stability can be prepared. Importantly, this result can be achieved without using hazardous monoisocyanates. Surprisingly, the mixed polycarbodiimide resins of the present invention also provide solvent resistance properties to crosslinked resins similar to polycarbodiimide resins made solely from aliphatic isocyanates.